Method and apparatus for regulating an output current from a power converter

ABSTRACT

Techniques are disclosed to regulate an output current through a load coupled to a power converter using a current source coupled to the load. For instance, one power converter according to the teachings of the present invention includes an energy transfer element coupled between an input of the power converter and an output of the power converter. A power converter controller is coupled to the energy transfer element to control a transfer of energy from the input of the power converter to the output of the power converter. A load is to be coupled to the output of the power converter. A current source is to be coupled to the load. The current source regulates the current flowing through the load to a threshold current value. The current source is coupled to sense a current through the load. A switch having a first and second end is also included. The first end is to be coupled to the load and the second end is coupled to the current source. The switch is coupled to be switched on and off at a duty cycle to control an average current through the load. The current through the load is substantially equal to a current through the switch and the current source.

REFERENCE TO PRIOR APPLICATIONS

This application is a continuation of US Application Serial No.12,167,091, filed Jul. 2, 2008, which is a continuation of U.S.application Ser. No. 11/086,600, filed Mar. 21, 2005, now U.S. Pat. No.7,414,862, entitled “Method and Apparatus for Regulating an OutputCurrent from a Power Converter.”

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates generally to electronic circuits, and morespecifically, the invention relates to a power converter.

2. Background Information

Power converters are often controlled to limit the average current to aload coupled to the output of the power converter. A typical example ofsuch a converter is a power converter used to charge a battery. Inbattery charger circuits of this type, the average current provided tothe load from the output of the power converter determines the timetaken to charge the battery. As well as controlling the charge time, itis important to regulate the average output current to the battery toensure the battery is being used within the specifications provided bythe battery manufacturer. Other examples of power converter circuitswhere the average output current is regulated are lighting circuits suchas those that deliver power to fluorescent tubes. In these circuits, theaverage current in the fluorescent tube is regulated in order to controlthe brightness of the fluorescent tube.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention detailed illustrated by way of example and notlimitation in the accompanying Figures.

FIG. 1 is a schematic of an example power converter adapted to regulatethe average current delivered to a load.

FIG. 2 is a schematic that illustrates a power converter outputincluding a current sense element and a current sense signal.

FIG. 3 is a schematic that illustrates a power converter outputincluding a switch that is coupled to the output.

FIG. 4 is a diagram illustrating examples of waveforms present in apower converter circuit.

FIG. 5 is a schematic that illustrates an embodiment a power converteroutput including a current source and a switch coupled to the output inaccordance with the teachings of the present invention.

FIG. 6 is a schematic that illustrates another embodiment a powerconverter output including a current source and a switch coupled to theoutput in accordance with the teachings of the present invention.

FIG. 7 is a schematic that illustrates an embodiment a power converterincluding a current source as a current sense element and a load currentregulator in accordance with the teachings of the present invention.

FIG. 8 is a diagram illustrating examples of waveforms present in anembodiment of a power converter circuit in accordance with the teachingsof the present invention.

FIG. 9 is a schematic that illustrates another embodiment of a powerconverter including a current source as a current sense element and aload current regulator including feedback time constant extension inaccordance with the teachings of the present invention.

FIG. 10 is a diagram illustrating examples of waveforms present inanother embodiment of a power converter circuit in accordance with theteachings of the present invention.

FIG. 11 is a schematic that illustrates yet another embodiment of apower converter with current source circuit as a sense element andintegrated circuit adapted to generate a feedback signal in accordancewith the teachings of the present invention.

DETAILED DESCRIPTION

Embodiments of a power supply regulator that may be utilized in a powersupply are disclosed. In the following description, numerous specificdetails are set forth in order to provide a thorough understanding ofthe present invention. It will be apparent, however, to one havingordinary skill in the art that the specific detail need not be employedto practice the present invention. Well-known methods related to theimplementation have not been described in detail in order to avoidobscuring the present invention.

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure, orcharacteristic described in connection with the embodiment is includedin at least one embodiment of the present invention. Thus, theappearances of the phrases “in one embodiment” or “in an embodiment” invarious places throughout this specification are not necessarily allreferring to the same embodiment. Furthermore, the particular features,structures, or characteristics may be combined in any suitable manner inone or more embodiments.

FIG. 1 is a schematic of one embodiment of a power converter in whichthe average output current is regulated. As shown, power converter 100includes an input coupled to receive an input voltage. In theillustrated example, the input voltage is 90 to 264 volts of alternatingcurrent (VAC). The AC voltage is rectified with diodes D1, D2, D3 and D4and is filtered and clamped with capacitors C1, C2 and C3, inductor L1,resistors R1 and R2, and diode D5. In the illustrated embodiment, powerconverter 100 is a switching converter including power convertercontroller circuit U1 coupled to energy transfer element, transformerT1. In the illustrated embodiment, power converter 100 includes anoutput circuit on the output side of transformer T1, where the output102 of the output circuit is coupled to a load 101. In one embodiment,power converter controller circuit U1 is a power converter controllerfrom the TinySwitch family of power converter controller circuitsavailable from Power Integrations, Inc. of San Jose, Calif. In otherembodiments, it is appreciated that other suitable types of powerconverter controller circuits may be utilized in accordance with theteachings of the present invention.

In one embodiment, power converter controller circuit U1 includes apower switch or transistor that is switched to regulate the transfer ofenergy from the input of the power converter to the output of the powerconverter in response to a feedback signal responsive to the output ofthe power converter. As shown in FIG. 1, a voltage, Vsense 103, isdeveloped across current sensing element, resistor R7, which isproportional to the magnitude of the current I_(LOAD) 105 flowing in theLOAD 101. This voltage 103 is applied across components resistor R8 andoptocoupler U2 as a current sense signal. This current sense signal iscoupled, through the optocoupler U2, to the power converter controllercircuit U1 on the primary side of the power converter. In anotherembodiment, the current sensing element could also be a currenttransformer or other suitable element to sense current in accordancewith the teachings of the present invention.

In the illustrated example, power converter controller U1 regulates thepower delivered to output 102 of the power converter in order tomaintain I_(LOAD) at the desired average regulation value. In theexample shown in FIG. 1, the power converter controller U1 regulates thepower delivered to the output of the power converter by skippingswitching cycles. The decision of whether to skip a cycle or not is madebased on whether the feedback signal received at the EN pin of powerconverter controller circuit U1 is above or below a threshold value setby the circuitry internal to power converter controller circuit U1. Thecircuit of power converter controller circuit U1 therefore regulates inresponse to a digital feedback signal. It is noted that a number ofother control techniques could be used such as pulse width modulator(PWM) current mode or PWM voltage mode, which would use for exampleanalog feedback signals. The power converter circuit example of FIG. 1also includes output voltage sensing using components transistor Q1 anddiode VR2 in order to regulate the maximum output voltage applied to theload 101.

FIG. 2 shows a simplified schematic 200 of another embodiment ofcircuitry that may be included at the output of a power converter toregulate the average current in the load. In the illustrated example,capacitors 202 and 207 may be considered as being equivalent tocapacitors C6 and C7, respectively, of FIG. 1. Current sense element 204may be considered as being equivalent to R7 of FIG. 1. Nodes 205 and 206may be considered as being equivalent to nodes 106 and 107,respectively, of FIG. 1. In the illustrated example, current sensesignal 203 is coupled to be received by a control circuit, which is usedto regulate the average current delivered to the load 201. In some lowcost circuits, capacitor 207 may be eliminated, which introduces moreripple voltage on the output across the load 201.

FIG. 3 is a simplified schematic of yet another embodiment of a circuit300 that may be included at the output of a power converter to regulatethe average current in the load. Compared to the embodiment illustratedin FIG. 2, a switch 305 has been introduced in circuit 300 coupled inseries with the load 301. In one embodiment, load 301 may include one ormore light emitting diodes (LEDs) coupled together as a chain of LEDs.The operating principle of circuit 300 is that the switch 305 is turnedon and off at a duty cycle appropriate to control the average lightoutput of the LED chain of load 301, while current sense element 304senses the current in the load 301 and coupled current sense signal 303to circuitry that regulates the output of the power converter to retaina constant LED current while switch 305 is on.

In one embodiment, an application for circuit 300 may be for use inembodiments of power conversion circuits that regulate not just theaverage output current to the load but also the instantaneous current.Furthermore, embodiments of such power conversion circuits control theinstantaneous current as well as the average current in the loadindependently.

In particular, one embodiment of a power conversion circuit inaccordance with the teachings of the present invention is used todeliver power to a plurality of LEDs coupled to the output of the powerconverter as the load. In this example embodiment, the instantaneous LEDcurrent is accurately regulated to maintain the correct spectral coloroutput from the LED when the LED is illuminated. The average LED current(and therefore light output) is regulated by turning the LEDs completelyoff for a percentage of the time. The ratio of the LED on time to offtime is referred to as the duty cycle. The frequency at which the LEDsare turned on and off in this way, is chosen such that the human eyecannot detect flickering as the LEDs turn on and off. In one embodimentthe on/off control frequency is between 200 Hz and 20 kHz. In oneembodiment, since the on/off duty cycle is used to control average LEDcurrent and therefore light intensity or output, the instantaneous LEDcurrent is controlled immediately when the LEDs are turned on to allowlinear control of the average LED current to low duty cycles inaccordance with the teachings of the present invention.

Example applications of where embodiments of power conversion circuitsmay be utilized include backlighting applications for liquid crystaldisplay (LCD) television (TV) displays, computer monitor panels, or thelike. In these types of applications, a white backlight may be generatedby using 3 chains of LEDs, one red, one green and one blue. Each chainof LEDs is coupled to a separate power conversion circuit output and isindependently controlled. The combined light output of the red, greenand blue LED chains is mixed to produce the desired white light. Sincethe individual light intensity and spectral output of each LED issensitive to the current flowing in it, very accurate control of thecurrent provided to each LED chain is provided as soon as the LEDs areturned on in accordance with the teachings of the present invention. Theaverage light output in these applications is controlled to generate thecorrect balance of the light contribution from each of the red, greenand blue LED chains so as to control the overall intensity of thebacklight, for user controller dimming for example, in accordance withthe teachings of the present invention.

Referring now back to the embodiment illustrated in FIG. 3, switch 305is illustrated in FIG. 3 as a mechanical switch. In another embodiment,a semiconductor switch such as a bipolar transistor or a metal oxidesemiconductor field effect transistor (MOSFET) may be utilized forswitch 305. However, the presence of capacitors 302 and 307, which limitpower supply output ripple voltage and noise within acceptable levels,may cause an undesirable operating characteristic.

During the period switch 305 is off, the output of the power conversioncircuit is completely disconnected, which results in a no-loadcondition. Under these conditions, the power conversion circuit outputvoltage will increase. The increase may be small and is undesirable, butsome increase will result due to the practical operation of powerconversion circuits. A consequence of this increase in power conversioncircuit output voltage when switch 305 is off is that when switch 305 isagain turned on, the voltage across capacitors 302 and 307 is in excessof that necessary to provide the required load current. Capacitors 302and 307 therefore discharge through the LED chain in load 301 causing aspike in the LED current each time load switch 305 turns on. Thedischarge current flowing through capacitor 307 is not even sensed bythe current sense element 304 and the power converter loop cannottherefore respond. However, even the discharge current flowing throughcapacitor 302, which the power converter loop does sense through senseelement 304, is not eliminated since the power converter cannot regulatethe energy that has already been delivered to the output capacitors.

Examples of waveforms present in the power converter circuitry areillustrated in FIG. 4. Waveform 401 of FIG. 4 illustrates that the powerconversion circuit output voltage rises during the period when the loadswitch is off, as shown with waveform 402. The load current 403subsequently overshoots when the load switch turns on causing thecurrent sense signal 404 to also overshoot above a current sensethreshold which would be set by the circuit designer as appropriate forthe particular type of LED being employed in the load. The powerconversion circuit responds at a rate dependent on the speed of thecontrol loop in the particular power converter causing the load current403 to drop below the required level before settling to the desired loadcurrent. As well as causing changes in the spectral output of the LEDs,this type of behavior also results in highly non-linear average currentregulation at low duty cycles of the load switch.

FIG. 5 shows one embodiment of a power converter circuit 500 inaccordance with the teachings of the present invention that allows theinstantaneous load current to be controlled without current overshoot.As illustrated, power converter circuit 500 includes an element 508coupled to switch 505. In one embodiment, element 508 is a currentsource circuit having a current threshold. The impedance of an idealcurrent source is zero when the current flowing through it is below thethreshold value and the impedance is adjusted when the threshold currentvalue is reach to maintain the current flowing at the threshold value.Though practical implementations of current source circuits onlyapproximate to these ideal characteristics, the operation introducesgreatly increased impedance when the current source current threshold isexceeded. For the purposes of the present disclosure, a current sourcecircuit is defined as any circuit where the impedance increases when acurrent threshold is reached in order to regulate the current flowing atthe current threshold value in accordance with the teachings of thepresent invention.

In the embodiment illustrated in FIG. 5, when switch 505 is turned on,the discharge current flowing through capacitors 502 and 507 and theload 501 increases until the threshold value of current source 508 isreached in accordance with the teachings of the present invention. Inone embodiment, the impedance and therefore the voltage dropped acrosscurrent source circuit 508 then increases as much as is necessary inorder to regulate the current flowing through the load 501 to thecurrent source circuit 508 threshold current value. The current sensesignal 503 is still proportional to the instantaneous current flowing incurrent sense element 504. In one embodiment, current sense signal 503is therefore coupled to be received by the power converter controller tobe used to regulate the load current after the initial capacitivedischarge current has been regulated by the current source circuit 508

In one embodiment, the current threshold of current source circuit 508is selected to be slightly above the normal current regulation value setin the main power converter control loop using current sense signal 503.As a result, the average voltage dropped and therefore dissipation incurrent source 508 is reduced in accordance with the teachings of thepresent invention.

FIG. 6 shows another embodiment of a circuit 600 in accordance with theteachings of the present invention. As shown in the depicted embodiment,circuit 600 includes a current source circuit 604, which serves as botha current source and current sense element in accordance with theteachings of the present invention. Due to the impedance characteristicsof current sources discussed above, when the current source threshold ofcurrent source circuit 604 is exceeded, the voltage across currentsource circuit 604 increases to regulate the current flowing throughcurrent source circuit 604. This is another form of current sense signal603, which can be used by the main power converter controller as thecurrent sense signal to regulate the current flowing through the load inaccordance with the teachings of the present invention.

In one embodiment, current source circuit 604 therefore regulates theinitial capacitive discharge current from capacitor 602 through the load601 at the time when switch 605 is first turned on and also provides thecurrent sense signal for the main power converter control loop inaccordance with the teachings of the present invention. This combinedfunction of current source circuit 604 is particularly effective wherethe current sense signal utilized by the power converter is a digitalsignal as is employed by controller U1 in the power conversion circuitembodiment illustrated in FIG. 1. However this configuration can also beused with a power conversion circuit employing an analog feedback signalin accordance with the teachings of the present invention. In oneembodiment, the output capacitor coupled directly to the load in theFigures described above is eliminated, since the current sense andinstantaneous current limiting function is performed by the samecircuit. This embodiment is therefore suited to applications whereincreased output ripple voltages may be acceptable.

FIG. 7 is a schematic illustrating an embodiment of a power conversioncircuit in which the feedback signal is analog. The schematic of FIG. 7shows a power converter with a current source circuit as a current senseelement and load current regulator. In particular, FIG. 7 shows thecomplete detail of the power conversion circuit primary and secondary,with load switch with drive circuitry (components Q206, Q207, R115 andQ208) and current source circuit (R110, Q205, RS101, R111, R112, Q204R109, Q203 and R108) as both current sense element and instantaneousload current regulator.

In the embodiment illustrated in FIG. 7, resistor R101 and diode U201provide a reference voltage (2.5V or 1.25V). Transistors Q201 and Q202form a differential amplifier to regulate the output voltage of thepower converter circuit of FIG. 7. When the base voltage of transistorQ202 is higher than the base voltage of transistor Q201, the diodecurrent of optocoupler U02 is high and the output voltage of the powerconverter circuit of FIG. 7 is controlled to be lower.

In the embodiment illustrated in FIG. 7, transistors Q205 and Q204 areused for controlling the load current. It is noted that the load shownin FIG. 7 is represented by a single LED. In other embodiments, the loadmay include a plurality of LEDs or another type of load in accordancewith the teachings of the present invention. When transistor Q208 isturned on, the voltage across resistor RS101 is substantiallyproportional to the load current and provides load current information,which is applied to the base of transistor Q204 via the resistor dividerformed with resistor R111 and resistor R112. Transistor Q204 controlsthe transistor Q205 base bias current to limit the transistor Q205emitter current by comparing the voltage across resistor RS101 with thevoltage between the cathode and anode of U201 and the differentialamplifier formed by transistor Q203 and transistor Q204. The voltage,Vsense, appearing at transistor Q205 may be considered as a currentsense signal that is therefore be regulated, thereby regulating thecurrent flowing in resistor RS101. The collector voltage of transistorQ205 is applied to the base of transistor Q202 via resistor R113,resistor R114 and diode D203 to provide a feedback signal throughoptocoupler U02 in accordance with the teachings of the presentinvention.

FIG. 8 illustrates example waveforms found in one embodiment of a powerconverter circuit in accordance with the teachings of the presentinvention. During the off time of the load switch, the output voltageincreases as described with relation to the example illustrated above inFIG. 4. Due to the use of a current source circuit as the load currentsense element as described above, the load current is a square wave, asshown in waveform 803 of FIG. 8, without the leading edge current spikeas compared to waveform 403 of FIG. 4. The LED current can therefore becontrolled linearly to very low duty cycles without the nonlinearitypreviously introduced due to the load current spike when the load switchis first switched on in accordance with the teachings of the presentinvention.

FIG. 9 is a schematic that illustrates yet another embodiment of acircuit in accordance with the teachings of the present invention. Thecircuit embodiment illustrated in FIG. 9 shares similarities with thecircuit embodiment illustrated in FIG. 7. The circuit embodimentillustrated in FIG. 9 includes the addition of diode D204 and capacitorC203. In the illustrated embodiment, capacitor C203 is peak chargedduring the initial voltage spike across transistor Q205, which furthersustains the feedback signal applied to the base of transistor Q202. Byfurther sustaining the feedback signal applied to the base of transistorQ202, transistor Q208, or the load switch, may be switched at very lowduty cycle during the normal off period to keep the output voltage fromincreasing as much during the switch off period of transistor Q208 inaccordance with the teachings of the present invention. This has thepositive influence of reducing the power dissipation in the currentsense element, or transistor Q205, at the beginning of the on period ofthe load switch, transistor Q208.

To illustrate, FIG. 10 is a diagram illustrating waveforms from anembodiment of a power converter in accordance with the teachings of thepresent invention. As shown in the waveforms of FIG. 10, the leadingedge spike in the current sense signal waveform 1004 is much smallerthan the equivalent spike in the current sense signal waveform 804 ofFIG. 8 in accordance with the teachings of the present invention. FIG.10 also illustrates the very short turn on pulses in transistor Q208during the ‘SW off’ period, as indicated with load switch drive signalwaveform 1002, which helps to reduce the voltage increase of the Voutwaveform 1001 during the load switch off period in accordance with theteachings of the present invention.

FIG. 11 is a schematic that illustrates yet another embodiment of acircuit in accordance with the teachings of the present invention. Thecircuit embodiment illustrated in FIG. 11 shares similarities with thecircuit embodiment illustrated in FIG. 9 in basic functionality. Howeverthe Vsense signal is no longer coupled to diode D204 but instead iscoupled to integrated circuit 1101. In the illustrated embodiment,integrated circuit 1101 is coupled to convert the Vsense signal receivedinto a quasi analog stepped output feedback signal 1102, which iscoupled through resistor R1113 to the base of transistor Q1102, whichcontrols the feedback current flowing through the LED of optocouplerU1102. In operation, the Vsense signal is compared to an internallygenerated threshold voltage VthL within integrated circuit 1101. WhenVsense is >VthL, output feedback signal 1102 is stepped down. If theVsense signal is <VthL, the output feedback signal 1102 is stepped up.In another embodiment, the polarity of output signal 1102 may beinverted with a modified circuit configuration in accordance with theteachings of the present invention. One advantage of using an integratedcircuit as shown in the illustrated embodiment is that storagecomponents such as for example capacitor C203 in FIG. 9 may be removed.Instead, when the Vsense signal falls to zero when switch Q1108 is off,the output feedback signal 1102 can be held at the same level. Inanother embodiment, the integrated circuit 1101 may also include switchQ1108 and the ON/OFF signal 1103 could then couple directly to theintegrated circuit 1103 in accordance with the teachings of the presentinvention.

In the foregoing detailed description, the methods and apparatuses ofthe present invention have been described with reference to a specificexemplary embodiment thereof. It will, however, be evident that variousmodifications and changes may be made thereto without departing from thebroader spirit and scope of the present invention. The presentspecification and figures are accordingly to be regarded as illustrativerather than restrictive.

1. A power converter, comprising: an energy transfer element coupledbetween an input of the power converter and an output of the powerconverter; a power converter controller coupled to the energy transferelement to control a transfer of energy from the input of the powerconverter to the output of the power converter, wherein a load is to becoupled to the output of the power converter; a current source to becoupled to the load, wherein the current source regulates the currentflowing through the load to a threshold current value, wherein thecurrent source is coupled to sense a current through the load, and; aswitch having a first and second end, wherein the first end is to becoupled to the load and the second end is coupled to the current source,wherein the switch is coupled to be switched on and off at a duty cycleto control an average current through the load, wherein the currentthrough the load is substantially equal to a current through the switchand the current source.
 2. The power converter of claim 1 wherein thecurrent source is coupled to the power converter controller to provide acurrent sense signal.
 3. The power converter of claim 2, wherein thecurrent sense signal is to be used to regulate a current through theload after an initial capacitive discharge current has been regulated bythe current source.
 4. The power controller of claim 2, wherein thecurrent threshold is selected to be slightly above the normal currentregulation value set in a main power converter control loop using thecurrent sense signal.
 5. The power converter of claim 1 wherein the loadcomprises one or more light emitting diodes (LEDs).